Motor vehicle headlamps typically include both a set of low beams for normal nighttime driving conditions and a set of high beams for such nighttime driving situations where the driver could benefit from additional forward-based illumination. In one common form, a single headlamp may include separate low beam and high beam filaments, while in another common form, separate headlamps each with their own dedicated filaments are provided. Regardless of the form, the beams are separately controlled in that the low beam filaments are energized in a low beam mode of operation while the high beam filaments are energized in a high beam mode of operation.
In older (i.e., on-off) headlamps, control of the (typically) tungsten-halogen filament operation included the use of relays and fuses, where one of the relays is used to energize the low beam filament at a predetermined maximum illuminating intensity, while another of the relays is used to energize the high beam filament, also at a maximum illuminating intensity. The fuses provide overcurrent protection for the filaments. These types of headlamps exhibit one of two modes, the first being completely off (i.e., no illumination) and the second being completely on that corresponds to complete illumination. It is difficult to dynamically change the luminance of these on-off configurations, as the relays do not respond to the type of continuous voltage supply needed to achieve graduated levels of illumination. Moreover, the fuses are prone to blowing and resulting in a non-operable headlamp until such time as the blown fuse can be replaced.
Furthermore, in situations where the high beams are activated in these on-off headlamp configurations, the low beams typically remain on as an additional illuminating source. Maximum illumination requirements (such as those imposed in the United States by the Federal Motor Vehicle Safety Standards (FMVSS, often referred to as FMVSS 108) often force the low beam illuminating intensity to be kept artificially low, even during periods of high beam inactivity. The FMVSS also includes provisions for the permissible height of vehicle headlamps, noting with specificity that such height is limited to between 0.559 meters and 1.372 meters. The problem of less-than-optimum low beam headlamp illumination is particularly acute in vehicles where the height of the headlamps is over 1 meter above the ground, such as those defined by truck, sport utility vehicle (SUV) and other enhanced-height vehicular platforms. As will be appreciated, many other countries, such as Japan and those in Europe, have adopted comparable rules and regulations to control the placement and illuminating intensities of vehicular lighting systems.
More recently, vehicular headlamps have been developed that provide the ability to deviate from a mere on-off functionality and instead have their illuminating intensity be adjustable; such a configuration is commonly referred to as a continuously variable headlamp or variable illumination headlamp. In one form of continuously variable headlamp, high intensity discharge (HID) headlamps (also referred to as xenon headlamps for the gas that is contained within the metal-halide lamps) use a voltage source that is passed through a ballast to both power the headlamps as well as provide a control signal to vary headlamp illumination intensity. In another form of continuously variable headlamp, the voltage from a voltage-modulated source passes through a high power field effect transistor (FET) or related headlamp driver circuit, where varying the duty cycle can cause a corresponding increase or decrease in the illumination intensity as needed. Such semiconductor-based switching operation of this latter form of continuously variable headlamp provides an increase in reliability over the conventional on-off headlamps discussed above. For example, and in addition to avoiding filament cycling problems, such solid-state switching reduces the need to have a fuse for each filament, which in turn eliminates fuse servicing requirements.
Regardless of the configuration, one problem associated with continuously variable headlamps and their control systems is the inability to adjust the illuminating intensity of the low beams in situations where enhanced range is desired. Accordingly, a need exists for an adjustable vehicular headlamp system and method for varying the light intensities of the low beam depending on the operational status of the high beam. A need likewise exists for providing greater illumination flexibility under a particular driving condition or environment to exploit the full illuminating capacity of the low beam headlamps while simultaneously remaining within the maximum guidelines established by the responsible transportation-related governmental body.